Itpa index

iTPA is the acronym for "intrathecal produced T. pallidum antibodies". It is used to assess neurosyphilis and is calculated as follows:

TPHA titre (CSF) / total IgG (CSF) × total IgG (serum) / TPHA titre (serum)

In the absence of antibody production against Treponema pallidum in the CNS, the ITpA index is 1 (0.5-2.0). A value > 2.0 indicates a specific antibody synthesis in the CNS, a value > 3.0 proves it with high reliability (sensitivity 84%, specificity 100%). If the ITpA is not calculated by ELISA but by titer levels, only a value > 4 is considered to be proof of autochthonous pathogen-specific antibody production due to the higher inaccuracy.

False-negative findings occur in secondary stage CNS infections and in vasculitic neurosyphilis.

Since the ITpA index and TPHA-AI (= modified ITpA index according to Reiber) only normalize over the course of years to decades after therapy, these antibody specificity indices (ASI) are not suitable as activity parameters.

1. Luger AF et al (2000) Significance of laboratory findings for the diagnosis of neurosyphilis. Int J STD AIDS 11:224-234.
2. Prange HW et al (1983) Relationship between neurological features and intrathecal synthesis of IgG antibodies to Treponema pallidum in untreated and treated human neurosyphilis. J Neurol 230:241-252.